This invention relates generally to fluid separation, and more particularly to fluid separation with recycle means.
Sorbent filters, such as carbon filters typically composed of one or more carbon beds, have been used for many years to remove offensive or harmful organic compounds entrained in air. Although the compounds are retained in all parts of the sorbent beds, studies have shown that a sorbable gas normally demonstrates earliest breakthrough (i.e., complete forward progression through the sorbent) first in the low velocity regions. In a sorbent bed of uniform dimensions and distribution of sorbent, such breakthrough occurs near the bed's edge zones or near a protrusion through the bed.
The finding of an early breakthrough pattern near boundaries, such as edge regions of a sorbent bed, may be explained based on the principles of shearing forces, kinetic energy, and surface area of sorbent pores and research related thereto. A carbon bed, for example, is composed of numerous carbon particles, typically activated carbon particles, confined within a defined volume. The carbon particles forming the bed have numerous tortuous pores of various diameters, generally defined as macropores (>50 nanometers in diameter), mesopores (˜2 to 50 nanometers in diameter) and micropores (<2 nanometers in diameter). Macropores and mesopores serve as primary transport channels. If a sorbent bed is considered having substantially planar upstream and downstream surfaces that are also substantially parallel to one another and designed to have a gas flow through the bed in a direction normal to the bed's surfaces, the sorbent bed may be characterized as having a central region or midsection in which most of the efficient sorption takes place and a surrounding peripheral region at the edge of the filter bed, proximate the outer wall of the bed or sorption chamber in which the sorbent is disposed. Studies have shown that most separation of gaseous components takes place in the micropores where shearing forces created by rapidly moving gases are not sufficient to cleave such compounds, in contrast to the mesopores and macropores, the primary transport channels, particularly in the midsection of a filter bed, where such shearing forces are sufficient to cleave sorbable compounds from carbon. Likewise, it is believed that a minimum threshold level of forced flow kinetic energy (KE, expressed mathematically as KE=½ mV2, where m is mass and V is velocity in consistent units) is required to open the micropore channels, form streamlines through them, and cause sorbable molecules to penetrate the pores deeply before their movement is stopped. Below such a level, sorbable molecules will instead simply continue to travel in forward directed macropores and mesopores. Thus, at low velocities and corresponding weak shearing forces near the edge regions of the sorbent bed, sorption takes place primarily in macropores and mesopores.
The rate of forward progression of the penetration front is determined by the relative rates of filling of the micropores in the midsection and the macropores near the boundary. Sufficient kinetic energy in the midsection enables sorbable compounds to deeply penetrate the micropores. In the midsection of the sorbent bed, the penetration front will advance at a given rate. Simultaneously, near the edge portions of the sorbent bed, where kinetic energy and shearing forces fall off markedly, sorbable compounds bind to macropore wall surfaces at a different rate than to micropore wall surfaces. Due to the much larger (orders of magnitude) surface area of micropores compared to macropores, the penetration front of the gas can advance more rapidly in edge zone macropores than in midsection micropores. Accordingly a penetration front bulge, and concomitant breakthrough, can occur near the edge of the sorbent bed. As a result, the edges or peripheral region demonstrates less capture efficiency (i.e. weight of contaminant captured by the sorbent/weight of sorbent) than in the midsection. Hence, earliest breakthrough generally occurs at the edges, or peripheral regions, of the bed.